The present invention relates to a tomographic imaging apparatus utilizing a magnetic resonance phenomenon, and more particularly to an MRI method and apparatus which are well suited to provide an inexpensive system and to produce a tomographic image of high quality.
In prior-art magnetic resonance imaging apparatuses (hereinbelow, abbreviated to "MRI apparatuses"), great efforts have been expended on enhancing the spatial homogeneity of a static magnetic field, as discussed in a paper entitled "Superconducting Magnet for Magnetic Resonance Imaging," Proceedings of the Japan Institute of Electricity, Vol. 106, No. 10, pp. 23-26 (1986).
Enhancing the homogeneity of a static magnetic field is required in order to reduce various distortions of an obtained image and to raise the measurement accuracies of parameters expressive of the features of a biological tissue, called "T.sub.1 and T.sub.2 relaxation times."
However, as the homogeneity is enhanced more with the prior art, the maximum amplitude of a nuclear magnetic resonance signal (hereinbelow, abbreviated to "NMR signal") which develops from an object to-be-examined becomes larger, and the dynamic range thereof becomes wider. As a result, the characteristic of an amplifier in the MRI apparatus and a requisite for the bit length of A/D conversion become severe, to give rise to the problems that increase in the cost of the MRI apparatus is incurred and that a quantization error ascribable to an A/D converter degrades the signal-to-noise ratio of an image. These problems become conspicuous especially in case of three-dimensional imaging.